An organic light emitting diode (OLED) is a light-emitting diode (LED) in which the emissive electroluminescent layer is a film of organic compounds which emit light in response to an electric current. This layer of organic semiconductor material is situated between two electrodes in some cases. Generally, for example, at least one of these electrodes is transparent. OLEDs sometimes are used in television screens; computer monitors; small or portable system screens such as those found on mobile phones and PDAs; and/or the like. OLEDs may also sometimes be used in light sources for space illumination and in large-area light-emitting elements. OLED devices are described, for example, U.S. Pat. Nos. 7,663,311; 7,663,312; 7,662,663; 7,659,661; 7,629,741; and 7,601,436, the entire contents of each of which are hereby incorporated herein by reference.
A typical OLED comprises two organic layers—namely, electron and hole transport layers—that are embedded between two electrodes. The top electrode typically is a metallic mirror with high reflectivity. The bottom electrode typically is a transparent conductive layer supported by a glass substrate. The top electrode generally is the cathode, and the bottom electrode generally is the anode. Indium tin oxide (ITO) often is used for the anode.
FIG. 1 is an example cross-sectional view of an OLED. The glass substrate 102 supports a transparent anode layer 104. The hole transmitting layer 106 may be a carbon nanotube (CNT) based layer in some cases, provided that it is doped with the proper dopants. Conventional electron transporting and emitting and cathode layers 108 and 110 also may be provided.
When a voltage is applied to the electrodes, the charges start moving in the device under the influence of the electric field. Electrons leave the cathode, and holes move from the anode in opposite direction. The recombination of these charges leads to the creation of photons with frequencies given by the energy gap (E=hv) between the LUMO and HOMO levels of the emitting molecules, meaning that the electrical power applied to the electrodes is transformed into light. Different materials and/or dopants may be used to generate different colors, with the colors being combinable to achieve yet additional colors.
There currently are several obstacles to continuous manufacturing processes for low-cost large-scale OLED production, e.g., for lighting applications. One such obstacle is the presence of local defects that cause electrical shorts. Some shorts are relatively benign in that they “burn-out” during operation resulting in only a small non-emissive, non-conducting area where the short was present. However, some do not burn out and additional shorts develop over time. These defects can be catastrophic, since current flows through the short rather than the working areas of the device. This may be true even for pixilated device architectures when all pixels are simultaneously energized, which may, for example, be the case for lighting applications. Causes of shorting defects include particle contamination during fabrication, asperities from electrode roughness, and non-uniformities in organic layer thickness.
For all of these mechanisms, the chance of encountering a defect increases as device area increases. This is particularly problematic for lighting applications where device sizes on the order of square meters are envisioned.
Some attempts have been made to incorporate an optional planarization layer between the ITO-based anode and the organic layers. However, the addition of a further planarization layer can complicate the production processes and increase expenses, e.g., as a result of the additional time and/or materials associated with the additional layer. Furthermore, the inventors of the instant application have determined that 30-40 nm defects below the organic layers can grow in size, e.g., such that they ultimately produce micron-scale defects. Thus, planarization layers may have to be comparatively thick to be effective, and the added thickness may have an impact on the operation of the anode.
Another obstacle to achieving large area devices results from the fact that OLEDs are current-driven, i.e. brightness scales with current density. Thus, larger devices require a greater current to be spread throughout the active area. There is a resistance to this spreading, given the finite conductivity of the electrodes. The finite conductivity of the electrodes can be quantified in terms of the voltage drop (Vd) as current travels along the relevant length (L) of the active area. Assuming a rectangular emitting geometry, for example, it can be shown that this voltage drop can be expressed approximately in terms of the average brightness (B), current efficiency or permittivity (E), and electrode sheet resistance (Rs) as follows:
  B  =            E      ×              V        d            ×              R        s                    L      2      
In this equation, the brightness (B) is expressed in Cd/m2, current efficiency or permittivity (E) is express in F/m2, sheet resistance (Rs) is expressed in ohms/square, and L is the units of length of scale of the device. It is noted that the voltage drop across the device extent (Vd) should be less than the actual intrinsic voltage across the OLED device (Vi), so as to turn it on for the specific current flow providing the brightness. In other words, in order to maintain efficiency and brightness uniformity, Vd ideally should be significantly less than the intrinsic voltage Vi required across the thickness of a device to attain a specific brightness. The electrode sheet resistance of a typical OLED is dominated by that of the ITO transparent conductor to a value of ˜10 ohms/square. The brightness required for most lighting applications is on the order of 1000 cd/m2. The most efficient OLEDs demonstrated to date at this brightness are green devices and these require an intrinsic voltage of ˜4V and have a current efficiency of ˜70 cd/A. Substituting these values into the above equation, one can see that, even for the most efficient OLEDs, the current spreading length should be less than ˜5 cm to keep Vd less than 10% of Vi. Again, this is a significant issue when device sizes on the order of square meters are envisioned.
Thus, it will be appreciated that there is a need in the art for improved techniques for making OLED devices that over these and/or other difficulties.
One aspect of certain example embodiments relates to planarizing one or more of a supporting substrate, an optical out-coupling layer stack system, and an anode layer, e.g., using one or more ion beams. Doing so may advantageously reduce the overall surface roughness of, and/or the presence of potentially short-forming peaks on, the subassembly on which subsequent organic layers are to be formed.
Certain example embodiments relate to a method of making an organic light emitting diode (OLED) device. A first layer comprising a first transparent conductive coating (TCC) is disposed, directly or indirectly, on a glass substrate. An outermost major surface of the first layer comprising the first TCC is planarized by exposing the outermost major surface of the first layer comprising the first TCC to an ion beam. Following said planarizing, the first layer has an arithmetic mean value RMS roughness (Ra) of less than 1.5 nm. A hole transporting layer (HTL) and an electron transporting and emitting layer (ETL) are disposed, directly or indirectly, on the planarized outermost major surface of the first layer comprising the first TCC. A second layer comprising a second TCC is disposed, directly or indirectly, on the HTL and the ETL.
Certain example embodiments relate to a method of making an OLED device is provided. An optical out-coupling layer stack (OCLS) system is disposed, directly or indirectly, on a glass substrate. A first layer comprising a first transparent conductive coating (TCC) is disposed, directly or indirectly, on a glass substrate. An outermost major surface of the first layer comprising the first TCC is ion beam treated in connection with an ion beam operating in an argon-inclusive environment at at least partial vacuum and at a voltage of 1275-1725 volts and an argon flow rate of 289-391 sccm. A hole transporting layer (HTL) and an electron transporting and emitting layer (ETL) are disposed, in this order, directly or indirectly, on the ion beam treated outermost major surface of the first layer comprising the first TCC. A second layer comprising a second TCC is disposed, directly or indirectly, on the HTL and the ETL.
Certain example embodiments relate to an OLED device. A substrate has an ion beam planarized major surface. An optical OCLS system is supported by the planarized major surface of the substrate, with the OCLS system having an ion beam treated major surface. An anode layer comprising indium tin oxide (ITO) is supported by the ion beam treated major surface of the OCLS system, with the anode layer having an ion beam treated major surface. First and second organic layers are supported by the ion beam treated major surface of the anode layer. A partially reflective cathode layer is supported by the first and second organic layers. The anode layer, following ion beam treatment, has an arithmetic mean value RMS roughness (Ra) of less than 1.2 nm, and is substantially free from peaks of greater than 40 nm in height.
According to certain example embodiments, the OLED device may be built into a lighting system or other electronic device such as, for example, a display or display device.
These and other embodiments, features, aspect, and advantages may be combined in any suitable combination or sub-combination to produce yet further embodiments.